A lot of that also depends on the powder and its characteristics. For years on top of years, in various big bore cartridges, 45/70, various larger capacity 458s and even .500s, I have used IMR 4198 low doses and never had an issue, and always had more air than powder....... I have used TINY doses of Unique in various cartridges, 223, 308 and so forth for subsonic loads, zero issues and I mean tiny nothing loads...... Don't ask me how or why, but no issues........ even give consistent velocities and pressures.................
But, all powders don't behave that way. LilGun "May" be one of those, because I have seen LilGun do strange things. I use LilGun quite a bit, it is THE POWDER for all my Super Short Big Bores, it is the Powder of choice for 7.62X40 that I shoot...... and great in many other areas, but I have seen it do strange things when compressed to much...... LilGun is probably fine in the little 300 BLK for subsonic or reduced velocities....... I could not see it being a problem,
I can answer your question on some powders, but I have not reduced LilGun to those amounts, and or many other powders....... I can say you can do about anything you want with IMR 4198 and most big bores........I have also used reduced loads with Unique, Red Dot, Blue Dot and those sorts of powders without issue.............
I would have to look again at the powders used in 450 Bushmaster, I have no experience with it, but I bet good money it would be easy to work with and reduce to most any level...........